Structure and Surface Properties of Stable Austenitic Steel Subjected to Liquid Carburizing at Lowered Temperature

Abstract

The paper studies the structure, chemical and phase composition, microhardness, and surface roughness of heat-resistant chromium–nickel (in wt %: 24.27 Cr and 18.81 Ni) austenitic steel subjected to liquid carburizing at a temperature of 780°С. It is established that the microstructure of the carburized layer predominately consists of carbon-rich austenite (γ-phase), chromium carbide Cr₇C₃, and cementite Fe₃C. It is revealed that carbides precipitate both at boundaries and inside the austenite grains; as we move away from the steel surface, the amount and dispersity of intragranular carbides decreases. It is also established that liquid carburizing leads to an increase in the microhardness of steel surface from 200 to 590 HV0.0025. The total depth of hardening is approximately 200 μm, and the hardened layer is gradient-wise. The surface of the carburized steel is characterized by large surface roughness (Ra = 2.40 μm and Rz = 17.60 μm), compared to the electropolished surface of specimens before carburizing (Ra = 0.17 μm and Rz = 1.80 μm), which is caused by several factors, including, e.g., oxidation of the surface.